This invention relates to polymeric films and particularly to those which provide a high oxygen barrier. This invention is especially directed at those films which are exposed to moisture at some point during their expected use life.
It is known that ethylene vinyl alcohol copolymers (EVOH) act as an oxygen barrier in multiple layer films. It is also known that vinylidene chloride copolymers (VDC) provide oxygen barrier properties. However, the oxygen barrier properties of the VDC and EVOH copolymers are affected differently by the humidity of the environment in which the polymers are used. The VDC copolymer provides relatively less oxygen barrier, compared to EVOH's having low ethylene content, and in some cases higher ethylene content, but it is relatively insensitive to changes in humidity. Namely, its oxygen barrier properties are generally about the same at high humidities as they are at low humidities. While there are some variations in the oxygen barrier as the humidity changes, those variations are fairly modest.
Low ethylene content EVOH provides a much better oxygen barrier than VDC at low humidity. Higher ethylene content EVOH provides better oxygen barrier than some VDC copolymers at low humidity. However, the EVOH's are quite sensitive to the humidity to which they are exposed, such that at high humidities, for example over about 75%-80%, the oxygen barrier properties of EVOH are less effective, and are thus less desirable than those of the VDC copolymer. Namely, at humidities over about 75%-80%, the oxygen transmission rates of those EVOH's are typically higher than the oxygen transmission rates of VDC copolymer.
Oxygen transmission rates, as disclosed are discussed herein are based on comparing 1 mil of EVOH against 1 mil of VDC copolymer, except where noted otherwise.
Thus while low ethylene content EVOH is generally preferred for use in low humidity environments, VDC copolymer is typically considered to be better for film which will be exposed to high humidity environments.
Higher ethylene content EVOH's have somewhat higher oxygen transmission rates than either low ethylene content EVOH or some VDC copolymers. The higher ethylene content EVOH has an intermediate moisture sensitivity. Since EVOH is generally easier to process than VDC copolymer, then in some cases, the higher ethylene content EVOH's are preferred in conventional use.
However, it has generally been the practice to use VDC copolymers for films which will be exposed to high humidity environments over some period of their use life, in order to gain at least the barrier properties attendant VDC copolymer, while avoiding the higher oxygen transmission rates of EVOH copolymer in the higher moisture environment. On the other hand, where consistently low humidity is expected on the outside of the package, the lower oxygen transmission rate of low ethylene content EVOH is generally preferred, for achieving extended shelf life.
One disadvantage of this practice has been that a package which is exposed to higher humidity for only a part of its use life has conventionally used a VDC copolymer barrier layer, and thus has not been able to benefit from the lower oxygen transmission rates of low ethylene content EVOH during those periods of its use life when the relative humidity is low enough that the package and its contents could benefit from the use of EVOH copolymer rather than the VDC copolymer. It would therefore be desirable to make a film which provides the high oxygen barrier of EVOH at lower humidities, and wherein its oxygen barrier properties are less sensitive to humidity variations, and are especially less sensitive to high humidity. Indeed it would be desirable to have a film which has the benefits of the lower oxygen transmission rate of low ethylene content EVOH at lower humidity while also having the advantage of the more stable oxygen barrier provided by VDC copolymer at higher humidities.
Thus this invention is directed at multiple layer polymeric films which have low oxygen transmission rates corresponding to the barrier provided by EVOH at low humidities, in combination with oxygen transmission rates corresponding generally to the barrier provided by VDC copolymer at high humidities. The films of the invention are capable of providing at least the barrier properties of the VDC copolymer at all times, and advantageously can provide the better barrier properties of the EVOH during periods when the relative humidity is lower than, for example, about 75%.